Cisco 2950G 24 Software Configuration Guide - Page 352
Q Configuration Considerations, Function
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Configuring VLAN Trunks Chapter 14 Configuring VLANs To avoid this, you should configure interfaces connected to devices that do not support DTP to not forward DTP frames, that is, to turn off DTP. • If you do not intend to trunk across those links, use the switchport mode access interface configuration command to disable trunking. • To enable trunking to a device that does not support DTP, use the switchport mode trunk and switchport nonegotiate interface configuration commands to cause the interface to become a trunk but to not generate DTP frames. Note On GigaStack GBICs, dynamic trunking is only supported when only one port of a GigaStack GBIC is being used. If trunking is required on a GigaStack GBIC where both ports are in use, use the switchport mode trunk and switchport nonegotiate interface configuration commands on both GBIC interfaces to cause the interfaces to become trunks. Table 14-4 Layer 2 Interface Modes Mode switchport mode access switchport mode dynamic desirable switchport mode dynamic auto switchport mode trunk switchport nonegotiate Function Puts the interface (access port) into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The interface becomes a nontrunk interface even if the neighboring interface is not a trunk interface. Makes the interface actively attempt to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto mode. The default switch-port mode for all Ethernet interfaces is dynamic desirable. Makes the interface able to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk or desirable mode. Puts the interface into permanent trunking mode and negotiates to convert the link into a trunk link. The interface becomes a trunk interface even if the neighboring interface is not a trunk interface. Prevents the interface from generating DTP frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link. 802.1Q Configuration Considerations 802.1Q trunks impose these limitations on the trunking strategy for a network: • In a network of Cisco switches connected through 802.1Q trunks, the switches maintain one instance of spanning tree for each VLAN allowed on the trunks. Non-Cisco devices might support one spanning-tree instance for all VLANs. When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switch combines the spanning-tree instance of the VLAN of the trunk with the spanning-tree instance of the non-Cisco 802.1Q switch. However, spanning-tree information for each VLAN is maintained by Cisco switches separated by a cloud of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud separating the Cisco switches is treated as a single trunk link between the switches. • Make sure the native VLAN for an 802.1Q trunk is the same on both ends of the trunk link. If the native VLAN on one end of the trunk is different from the native VLAN on the other end, spanning-tree loops might result. 14-16 Catalyst 2950 Desktop Switch Software Configuration Guide 78-14982-01